Data Network Telephony
Data networks such as the Internet are now being used to transmit voice. Such data-network-based telephony networks provide an alternative to public-switched telephone networks (“PSTNs”) for placing telephone calls.
FIG. 1 depicts a schematic diagram of a system 100 for conventional voice communications over a data network. The system includes data network 102 and public-switched telephone networks (“PSTN”) 120 and 122. The specifics of the architectures and communications protocols of such systems are not described herein except to note that they are quite different from one another such that direct communication therebetween is not possible without converting formats, protocols, etc. It will be appreciated that while two PSTNs (i.e., PSTN 120 and 122) are depicted, there is, at least functionally, only one worldwide PSTN. Communication between a PSTN and a data network is implemented via a “gateway.” In the general sense a gateway is an entrance to and an exit from a communications network. A gateway is typically an electronic repeater device that intercepts and translates signals from one network to another. A gateway often includes a signal conditioner that filters out unwanted noise and controls characters. In data networks, gateways are typically a “node” on both networks that connects two otherwise incompatible networks. Thus, gateways often perform code and protocol conversions. Such an operation would be required for communication between a PSTN and a data network. Assuming an analog voice signal is delivered from the PSTN, the gateway digitizes that signal from the PSTN and encodes it and transmits it as “packets” (hereinafter “digitized voice signal”) over the data network according to data network protocols. In other embodiments, the signal from the PSTN is a digital signal, such that analog-to-digital conversion is not required. Protocol conversion is still required.
An element associated with a gateway is a “gatekeeper.” A gatekeeper is responsible for gateway registration, address resolution and the like. A gatekeeper may be viewed as the router that directs a digitized voice signal to a “terminating” gateway (i.e., a gateway that provides protocol conversion for transmission over a PSTN, for example, to a telephone). As used herein, the term “gateway” includes both the gateway and gatekeeper functions.
System 100 therefore also includes gateway 110 that acts as a conduit between PSTN 120 and data network 102, and gateway 112 serving as a conduit between data network 102 and PSTN 122. The system further includes telephone 130 that is connected, via link L1, to PSTN 120 and telephone 136 that is connected, via link L8, to PSTN 122. The links that are depicted in FIG. 1 are, as is well known, trunk lines, trunk groups, etc., as appropriate.
In operation, voice message 140 from telephone 130 is transmitted over link L1 to PSTN 120. Within PSTN 120, voice message 140 is routed to switch S2 over link L2. Switch S2, the operation of which is well known in the art, will typically route voice message 140 to another switch (not shown) over a trunk group (not shown). In such a manner, voice message 140 moves through PSTN 120 being routed from switch to switch until it is carried over a final link L3 out of PSTN 120. Voice message 140 is then carried, over link L4, to gateway 110. “Originating” gateway 110 performs protocol conversion and digitizes, as required, voice signal 140. Voice message 140 is then routed (the gatekeeper's function) into data network 102. For clarity of presentation, the voice message will be assigned the same reference numeral (e.g., 140), notwithstanding the fact that the signal carrying the message is physically changed during transmission through the system.
Message 140 is transmitted over call path DNCP to “terminating” gateway 112 wherein the signal leaves data network 102. Note that the designation “originating” or “terminating” applies on a call-by-call basis. In other words, for a first call, a particular gateway can be an originating gateway, while for a second call, that same gateway can be a terminating gateway. Moreover, packets typically flow in both directions since both parties typically talk.
A call path through a data network, such as call path DNCP through data network 102, is not fixed according to a defined hierarchy as in a PSTN. Rather, an originating gateway “selects” a terminating gateway and the voice signal is routed by successive network elements (e.g., routers, bridges, etc.) through the data network to the terminating gateway. Since routing decisions are made by each network element, call path DNCP is not a priori known or set.
Gateway 112 receives voice message 140 and converts it to a form suitable for transmission through PSTN 122. Voice message 140 is delivered over link L5 to PSTN 122. Within PSTN 122, voice message 140 is routed via over links, such as link L6, to switches, such as switch S4. Voice message 140 is carried over link L7 out of PSTN 122 to link L8 to telephone 136 to complete the call.
Often there is more than one gateway 112 capable of terminating the call. An originating gateway may allocate calls to plural terminating gateways based on plural factors such as price, quality, availability, desire to equally distribute calls among terminators, etc. However, such allocations do not ensure that the system is always configured optimally, since all of the parameters effecting the allocation among terminating gateways change rapidly and somewhat independently of each other.